Electronic cross-point matrix telephone private switching equipment

ABSTRACT

A telephone private switching equipment of the space-division operated type comprises a matrix of electronic switch cross-points each made of a single controllable two-state member connected across a conversation conductor of a row and a conversation conductor of a column of the matrix and having a trigger terminal connected through unidirectional members to both a cross-point activation conductor of this row and a cross-point activation conductor of this column, each row and each column comprising only these two conductors. Each column of the matrix is controlled from a cross-point selection control unit capable of applying trigger pulses to the cross-point activation conductor of the column and service voltages and currents to the conversation conductor of the column. Each subscriber&#39;s line circuit of the equipment includes a two-wire to one-wire converter the one-wire terminal of which is connected to the conversation conductor of a row and includes means capable of applying periodical pulses to the cross-point activation conductor of this row. Each external network line connecting circuit of the equipment is connected by a one-wire conversation lead to the conversation conductor of a row comprising only a single cross-point of the matrix.

SHORT SUMMARY OF THE INVENTION

The present invention concerns improvements in or relating to electronictelephone private switching equipment adapted to handle traffic of about50 to 250 connections to the external telephone line network.

It further concerns such equipment which use conventionaltwo-conversation wire subscriber's sets which can be conventionallycalled by a ringing signal having the frequency of the electrical mains.

The invention particularly concerns such equipment which is operated ona space-division basis, the general organization of which is illustratedin FIG. 1 (a) of the attached drawings. According to this organization,the switching equipment comprises a logical control unit LOG which is intwo-way linkage with subscriber's line circuits AB, from O to n, andexternal network line connecting circuits ER, from 0 to q.Theintercommunications are established between the circuits by means of amatrix made of columns Bo, B1, ...,Bn, Bn+1, ... and of rows L0, L1, ..Lm, ... Lm+1, ... Lm+q and of electronic switch cross-points such as PCdefining the intersection points between these columns and these rows.Each row is connected to a circuit AB or a circuit ER. Each column isconnected to a cross-point selection control unit CB, from 0, 1, ..., n,n+1, ... each controlled from the logical control unit LOG for selectiveactivation of the cross-points PC on a X-Y addressing basis. The actualnumber of rows is determined by the number of subscribers in the privateswitching equipment and of external network lines connected to theequipment. On the other hand, the number of columns is defined by thedensity of the traffic which is required for the equipment.

Shortly summarized, the algorithm of operation of such an equipment maybe termed as follows, reference being made to the full line part of FIG.1(b) which illustrates such an algorithm:

The logical control unit LOG ensures a systematic permanent scan EXPL ofthe conditions of the subscriber's line circuits and of the externalnetwork line connecting circuits. This scan is executed in a loopedsequence with an automatic advance of the addresses allotted to the saidcircuits. Each step of the sequence tests the condition of thecorresponding circuit. When this TEST operation detects an occupiedcondition, a check order, v, is sent back to the EXPL routine which isstopped and enters into a check routine determining whether saidoccupied condition results from an actual communication connection orfrom a communication connection request. In this second alternative, theTEST issues an order yes which results in the allotment of a column ofthe matrix to the caller set or line circuit. Once this attributionphase ATB is over, inhibition of the EXPL phase is cancelled. Thescanning operation is resumed by the logical unit LOG. Simultaneously,the caller is requested to dial the number of the party he wishes tocall. During this phase NoI, the logical control unit LOG proceeds to asearch of the called party, subroutine RDm. When this party is occupied,the occupancy signal occ is send back to the calling party and the samesignal resets the phase ATBe. When the called party is not occupied, asignal lib is directed to the part of the logical control unit LOG whichattributes a column to the said called party, routine ATBr.

Such an equipment and such an algorithm necessitate a high number ofcross-points of the matrix and such cross-points usually are of arelatively high complexity, thereby necessitating the switching of arelatively high number of circuits as for instance: -- at least one pairof conversation conductors, a ringing current conductor, a call returnconductor and at least one dialling pulse conductor per row and percolumn. The cross-point proper usually comprises several "memory"solid-state switch members such as the ones known as "thyristors".

It is the object of the invention to provide an equipment adapted to becontrolled in a time and space division process as hereinabove describedwhich embodies a single switching matrix of a highly simplified designin that it permits a single two-condition electronic switch member percross-point for switching a single pair of row and column conductors.Each pair comprises a cross-point selection conductor and a conversationconductor and the conversation conductor of a column is also used in theswitching process of the electronic switch members which are connectedthereto.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in full detail with reference to theaccompanying drawings, wherein:

FIG. 1a shows the general organization of the prior art equipment of thetype to which the present invention relates;

FIG. 1b shows an algorithm for operation of the equipment of the typeshown in FIG. 1a;

FIG. 1c shows a modification of FIG. 1a when the invention is put intopractice. This modification consists in the omission of cross-points PCin the matrix between the external network line circuits ER and thecolumns of said matrix but a single cross-point per such externalnetwork line and a single one particular column is maintained. Forinstance, the single cross-point PC is maintained between line Mm+1 andcolumn Bn, the single cross-point is maintained between Lm+q and Bn+1etc.

FIG. 1d shows the algorithm of operation of the equipment of FIG. 1c;

FIG. 2 shows an illustrative embodiment of the cross-point selectioncontrol unit of a column of the matrix together with a partialrepresentation of the controlled column and of other columns of thematrix, included one column devoted to the access of an external networkline to the equipment according to the above stated subsidiary featureof the invention,

FIG. 3 shows an illustrative embodiment of a subscriber's line circuitof the equipment, adapted to be connected to a row of the matrix of FIG.2 and to cooperate with the cross-point selection control unit of thesaid FIG. 2, and,

FIG. 4 shows an illustrative embodiment of an external network lineconnecting circuit of the equipment, adapted to be connected to across-point of the matrix of FIG. 2.

DETAILED DESCRIPTION

The modification of the algorithm shown in FIG. 1d will be firstdescribed. It must be emphasized that this modification does notnecessitate in the logical control unit LOG any addition or modificationof the prior commonly used hardware.

In this modification, any access request DR from an external networkline connecting circuit temporarily inhibits the progression of the scanEXPL and, pursuant thereto, the circuits which control said EXPL.routine answer by activating means validating at VDR the column of thematrix wherein is connected the cross-point allotted to the saidexternal network line connecting circuit. This validation process givesaccess to either an operator's position, when provided in the equipmentor one of the subscriber's line circuit, which has been providedprivileged in this respect. The operator's or privileged position maycall through the routine NoI the subscriber's set requested from theexternal network line and consequently have this latter connected to thecolumn allotted to the external network line connecting circuit. When asubscriber of the equipment, after having received the attribution of acolumn, dials a number which begins by a code pointing to the externalnetwork, the detection of this doce Res blocks the prior attribution ofthe column at ATBe and substitute thereto the attribution of a column,routine ATBR, which possesses an access to the external network. Thisattribution routes the call to the external dialling facilities, routinemarked No EXT in FIG. 1b.

As concerns the hardware of the invention, as is commonly known, theLogic control unit LOG includes, inter alia, a clock controlled timebase circuit which generates the relatively shifted signals which areneeded for its own operation and for the control of the subscriber'sline circuits, the external network line connecting circuit and of thecross-point selection control units associated with the columns of thematrix. The main function of LOG is to attribute (or "distribute")columns to the calling parties and thereafter to execute the operationsuseful for the establishment of the requested communications and,finally, to control the connections between the subscribers and thecolumns.

Each subscriber's line circuit is provided with a multi-bit identifyingnumber and so are the external network line connecting circuits.

Each column of the matrix can only have two cross-points in simultaneousactivated conditions. Consequently, in the logics only two number storesare allotted for a column within the said logic, one store for storingthe number of the calling party and the other one for storing the numberof the called party. It is a decoding operation of the contents of thesestores which ensures the addressing of the activation controls of thecross-points connected to the columns through the cross-point selectioncontrol units of the columns. Also, the decoding operation is used foraddressing information exchanges between the AB and the ER circuits.

The establishment of a connection calls for the following means:

The central clock of LOG actuates a scanner which sequentially tests theconditions of the subscriber's line circuits. Each detected conditioncan only be one of two terms of an alternative:-- "hooked" or"unhooked", the words referring to the switchhook of the handset of thesubscriber (as translated to a terminal SLE of the subscriber's linecircuit of FIG. 3). It consequently necessitates only two markingvoltage conditions at a test terminal of the subscriber's line circuit.Each step of the scanner increments by one unit the content of a counterhaving as many counts as are subscribers in the equipment. At each step,consequently, this counter has a content corresponding to the binarycode address of the subscriber's set circuit the SLE terminal of whichis connected to the scanner.

When an "unhooked" condition is detected, the advance of the scanner andconsequently of the counter is stopped. The central logic must determineif this subscriber has already received an attribution of a column inthe matrix or not. The determination is obtained by a systematic searchin the number stores of the column attributing logics. If the binarynumber of the subscriber is found in one of these stores, it means thatthe subscriber is dialling or speaking to another party; if not, itmeans that a column must be attributed to the subscriber. The searchconsists of a sequential read-out of the stores and each read-out stepcontrols a comparator circuit the other code input of which is connectedto the binary code number on which the counter is stopped. During thesequence, when a code coincidence is detected, the "yes" output of thecomparator re-activates the scanner. At the end of the sequence, if nocoincidence has been detected, the binary code number of the subscriberset is copied into a store of a column control logic which is the first"free" one encountered by the column attributing logic which includes anaddress table in this respect. The scanner is re-started and thecross-point selection control unit of the thus selected column appliesto the subscriber's set circuit a dialling request signal. The callingsubscriber dials the number of the other party he needs and thecorresponding subscriber' set binary code number is stored in the"called party" store of the logics controlling the column which has beenattributed to the calling party. Thereupon, the central control logicLOG activates means for systematically and sequentially interrogatingthe subscriber's binary code stores of the columns logics and comparingtheir contents to the "called party" binary code number. When acoincidence occurs, the search is stopped and an occupation marking tonesignal is sent back to the calling party subscriber's set circuit. Whenno coincidence occurs during the systematic search, the call signal issent to the called party subscriber's line circuit. When the calledsubscriber unhooks his handset, the connection occurs through a secondcross-point of the column on which the calling party has already beenconnected by the activation of a cross-point between his row and theconcerned column.

When the calling subscriber begins to dial with a code part pointing toa request of attribution of an external network line, said code part isdecoded in the control logic LOG which answers by controlling thetransfer of the calling party binary code number from the column controllogic store wherein it was first copied into the corresponding callingparty binary code number store of a free column logics allotted to acolumn connected to an external network line connecting circuit. Oncethe transfer is over, the central logic LOG clears the previously storewherein was first stored the binary code number of the calling party. Itmust be understood that the above-defined code part pointing to arequest of attribution of an external network line does not point to anidentified one of such external lines. The calling party will not beinformed of such a transfer of attribution of a column and will receivethe normal answer signal except when the external line is alreadyoccupied, in which case he will receive back the occupancy tone signal.

The above analysis of the operation of the known means of the equipmentclearly shows that when, according to the present invention, each rowand each column of the matrix each only comprises a single conversationwire and a single cross-point activation wire, i.e., only one pair ofconductors, these pairs of conductors of a row and a column being onlyinterconnected by a single cross-point two-state electronic switch, eachsubscriber's line circuit must be specially provided with means forreceiving the ringing current, the dial pulses and the signal to requesta break of the communication, as these current and signals cannot berouted through the cross-points of the matrix. Further each subscriber'sline circuit, as well as each external network line connecting circuit,must be adapted to the one-wire conversation feature of the matrix.

According to a first subsequent feature of the invention, eachsubscriber's line circuit and each external network line connectingcircuit includes a two-wire to one-wire translator circuit the one-wireterminal of which is connected to the conversation conductor of a row ofthe matrix.

According to a second subsequent feature of the invention, eachsubscriber's set circuit includes a local ringing relay controlled by alocal store activated from the central logical unit and a local breakrequest signal generator controlled by a further local store activatedfrom the logic control unit of the equipment.

Referring now to FIG. 2, a cross-point in a matrix according to thepresent invention comprises, connected across the "vertical" (column)cross-point activation conductor sv and the "vertical" (column)conversation conductor cv, of the column to which it is connected, aseries circuit comprising a resistor r1, a un idirectional element d0and a resistor r0. The trigger electrode of a thyristor Th is connectedto the connection point between d0 and r0. The cathode of the thyristoris connected to the conductor cv. The connection point between r1 and d0is connected by a unidirectional element d1 to the cross-pointactivation conductor sh1 of the corresponding row of the matrix whereasthe anode of the thyristor is connected to the conversation "horizontal"conductor ch1 of the said row.

The unidirectional element d0 provides a separation between thecross-point activation control circuit and the conversation circuit. Theresistance r0 is the cathode-trigger bias resistor. The element d1transfers the negative ground which is received by the conductor sh fromthe subscriber's set circuit connected to the row when said circuit isunactivated. The resistor r1 transfers the triggering pulse from theconductor sv when activated.

In FIGS. 2 to 4, the following notations are used:--

OE = electronic zero voltage, actually equals -12 volts,

OCX = - 48 volts distributed throughout the equipment (the -48 volts ofthe supply of the subscriber's lines is noted -V),

+v = + 12 volts.

In each cross-point selection control unit CB, as for instance the unitCB₁, FIG. 2, which controls the cross-points (PC) o1, ..., (PC)11, ...,the cross-point activation conductor sv1 is connected through aunidirectional element d12 to the output of an amplifier (AMPI) whichreceives at the terminal IAT the trigger pulse for the thyristor of thecross-point which is at the same time instant selected at sh by thesubscriber's set circuit connected to the row of which sh is aconductor. This selection process will be herein later described.

The conversation conductor cv of each column is connected in the unit(CB) of the said column, to an electrical current source SCB under avoltage which is maintained at a constant value by the Zener Z2. Theelectrical current source is activated when the terminal PB of the unitreceives a voltage which develops a D.C column bias from the progressivecharge of a condenser circuit (MPB). PB receives said voltage at thetime instant of attribution of a column to a subscriber which requeststhe establishment of a communication. Conversely, the liberation of acolumn is obtained by the application to the terminal LB of the unit ofa signal ensuring the progressive discharge of the condenser of (MCB)through a transistor it unblocks. During the time interval of occurrenceof the signals PB and LB,the condenser C1 has maintained in activity theelectrical current source SCB. The D.C. voltage on cv is limited by thezener Z1 and a diode d10 so that the transistor CC coupling (MCB) to(SCB) is protected against any parasitic or stray action which may bedue to the voice frequency modulations on cv during the communication,and the electrical current from (SCB) is maintained at a valueunaffected by these modulations.

In order to avoid untimely triggering of the thyristors of the column,the control unit comprises triggerring preparation means for ensuring aprogressive rise of the cathode-anode voltage of the thyristors. Saidmeans comprises a circuit (MPA) having a condenser C2 which is chargedby the application of a voltage step to an input PA and having an outputcircuit SPA connected to the conversation conductor cv of the column.Once the triggering has occurred, a further voltage step applied at CAwill produce the discharge of C2 through a transistor TLD which isunblocked by the siad voltage step at CA. A diode d11 creates in SPA anadditional threshold which increases the immunity of the thyristorsagainst stray signals.

Through a further input terminal TON, the conductor cv may receive, whenneeded, musical frequency signals for dialling request, backcall returnand wait on occupied condition of the called party.

A further input I0 is so provided that an operator may enter in anestablished communication without disturbing the established connectionson the column. An operator's set is considered as a peripheral circuitin an equipment according to the invention. When provided, its functionwill be the normal one, i.e. it will receive all calls from the externaltelephone network and the operator will dial to the logical control unitLOG the local binary code number of the called party. The LOG unit willthen dealt with the incoming call as it handles any other callwithin theequipment. In order not to disturb the communication on an operator'sintervention, the activation of I0 will produce a variation of thethyristor bias current on cv which will compensate for the introductionof a dummy impedance simulating a third cross-point in the communicationcircuit.

The pair of conductors sh and ch of a row is connected to respectivewires of a subscriber's line circuit such for instance as the circuit(AB1) shown in FIG. 3.

In this circuit, the conductor sh is connected to the output of atransistor SCTh which is blocked each time a temporary circuitvalidation signal is applied from the binary code number decoder of thescanner of LOG to the terminal VDAB₁. When the transistor SCTR isblocked, the conductor sh is no more connected to the OCX voltage. Ifduring such a condition, a trigger pulse from CB triggers the thyristorin its active state, the electrical current from the biassing source SCBthereafter maintains the thyristor "conducting" and the cross-point PCconnects the conversation conductor ch to the conversation conductor cvof the column.

Of the subscriber's handset connected to the circuit AB1 are only shown,at PAB1, the commutator switch of the handset cc and the dialling pulsecontact cd. The wires of the handset ends in a two-wire to one-wireconverter T, hence the required conversion to the one-wire terminal Ch1to be connected to the conversation conductor ch1 of the matrix row. Onthe one-wire side, the winding of the translator is shunted by a voltagelimiter made of the two reversely connected zeners Z3 and of a pair (D)of the opositely connected unidirectional shunts. In the wire f isinserted a contact of a relay K2 having a shunt diode across its coil,D21, fed through a transistor having its emitter to the OE voltage. Thebase of the transistor is connected to the output of a gate GA of theNOR kind, one input of which is connected to the output terminal of astore, (MA) of the bistable type. Each control input of (MA) receives aninput from an AND-gate unblocked when the signal VDAB1 is present. Thestore (MA) marks a call to the subscriber when, during a period ofpresence of VDAB1, a voltage is applied to the input AP. The reset ofthe store is similarly ensured by application of a reset voltage to theinput terminal AA. When K2 is energized, it applies the ringing currentunder 70 Volts on the converter T, contact of K2 at work so that theA.C. current is applied to the ring of the unshown subscriber'stelephone set. On the other hand, when K2 is unactivated, its contact,in the rest position which is shown in the drawing, the double shunt (D)protects the converter against any overvoltage which could be due to theringing circuit at the time instant of the reset to rest of the contactof K2. A zener Z5 and a thermistor RT respectively ensure a voltage anda current limitation on the handset's side, a condenser associated withthe zener and the thermistor enabling the transmission of the lowfrequency modulation to the subscriber.

On the subscriber's side, the winding of the converter T is divided intotwo half-windings connected by a condenser; one half-winding isconnected to ground and the other one is connected to -V through thecoil of a relay K1. The actuation circuit of this relay K1 passesthrough the loop of the subscriber's set, which closes through thecommutator switch cc and the dialling contact cd. When the subscriberunhooks, cc closes the loop, the relay K1 is actuated and closes thecircuit which, at its contact k1, applies the voltage +v to the gate GAand to an output NAND gate GLE connected to the terminal SLE, through adiode d20, which must be understood as being the terminal the conditionof which marks the condition of the subscriber's set for the conditionscanning means of LOG. The gate GLE is unblocked at each application ofa test voltage at VDAB1 provided a bistable store (MIR) is at rest. Whenthis stor (MIR) is actuated from the applicaton of a signal at theterminal IR which is connected to its set input through an AND-gatecontrolled from VDAB1, GLE is blocked, but, through the NOR-gate GT, thetonality signal permanently applied at TONIR to the circuit istransmitted through the transistor IGT to the converter T andconsequently applied to the unhooked subscriber's set.TONIR is atonality signal requesting the subscriber to break the communication bydepositing his handset on its hook. On the side of the matrix, thecircuit comprises an impedance adapter stage Te which is both de-coupledagainst the voice frequencies and shunted by a constant D.C. generatorTLN. The zener Z4 ensures the constant feature of the current. When thesubscriber actually replaces his handset on the hook, the stor (MIR) isreset by the voltage +v passing through the OR gate GRz at the timeinstant when VDAB1 is applied to the circuit.

As the dialling through contact cd opens and closes the subscriber'sloop, the relay K1 follows the dialling and the dial pulses aretransmitted from SLE to the logic control unit LOG.

Unhooking operation of the subscriber interrupts a ringing signalthrough the gate Ga from the closure of k1 before the resetting of (MA)by the reset signal is applied to AA.

Turning back to the cross-point column control unit CB, TEF is an inputterminal adapted to ensure the connection of a "dummy thyristor" to theconversation conductor cv1 of the column. Two thyristors will benormally connected to each column during a communication in activated orconducting conditions thereof. The input terminal TEF enables thesimulation of the existence of a second conducting thyristor as long asone only is so connected to the column, after a connection request butprior to the satisfaction of this request of a subscriber. This "dummythyristor" connection balances the voltages and currents in the matrix.

In equipment such as herein above described, the establishment of acommunication between two subscribers may be stated as follows:

A subscriber wishes a communication with another subscriber of theequipment. He raises his handset from its hook and consequent markingoccurs at the terminal SLE of the subscriber's set circuit AB. At thefirst passage of the scanner on this terminal, VDAB is validated and thelogical control unit LOG stops the scanning and operates for attributinga column to the subscriber by applying to the terminal PB of thecross-point control unit it selects the signal controlling theapplication of the bias voltage to the conversation conductor cv of itscolumn. All thyristors connected to this conductor are consequentlybiassed. Thereafter, the unit receives the voltage preparing thetriggering of a cross-point thyristor, on the terminal PA. When theterminal VDAB of the subscriber's set circuit is reactivated, a triggerpulse is simultaneously applied to IAT. The selection is operated andthe thyristor of the cross-point connected to the row to which thecalling subscriber's line circuit is connected is turned conducting.

The tonality signal requesting the thus connected subscriber to dial thenumber of the subscriber's set he wishes to reach is applied to the TONinput of the cross-point activation control unit. The calling subscriberdials and if the called party is not occupied, at the nest passage ofthe scanner on the called party subscriber's set circuit, the process isrepeated in that another triggering pulse is send to the conductor sh ofthe same column and the thyristor of the cross-point of said column towhich is connected the row of the called subscriber's line circuit isturned conducting. Up to this second trigger pulse, the input TEF of theconcerned CB unit had been activated for the simulation of a dummythyristor and the second trigger pulse de-activates this terminal TEF.The call is applied on the terminal AP of the called party subscriber'sline circuit, stored into (MA), ringing occurs. When the calledsubscriber unhangs his handset from the hook, the relay K1 modifies thevoltage at the terminal SLE of his circuit at the next passage of thescanner on this circuit. Ringing is stopped and thereafter the store Mais reset by the reset voltage which is applied to the terminal AA. Whenthe communication is over, a liberation control voltage is applied tothe terminal LB of the column contol unit involved in this connectionand, as above explained, the electrical current source (SCB) is turnedoff and the thyristors consequently turn off.

After the second triggering pulse occurence, a control voltage wasapplied to CA for cutting the voltage which prepared the triggering ofthe thyristors. When said voltage is applied as soon as the occurrenceof the first trigger pulse, then the preparation of trigerring isrepeated before the second occurrence of the triggering pulse.

When an operator's position is provided in the equipment, and theoperator wishes to intervene in an established communication, theterminal I0 of CB receives an additional current of a suitable value totemporarily implement the current already applied to the conductor cv ofthe column so that three cross-point thyristors can be maintained inconducting condition on this column. The operator's position possesses aspecial row of the matrix on the conductors of which the siad operatorcan manually apply the suitable voltages for forcing the selection andtriggering of the thyristor which, in said row corresponds to the columnalready occupied by the communication.

The connection of the equipment to the external network lines is ensuredthrough appropriate line connecting circuits connected to specializedcross-points on certain columns of the matrix such as is the PCRcross-point illustrated on FIG. 2. The circuitry of such a cross-pointis similar to any other cross-point except that the conversationconductor of the row is connected at CHj to a single external networkline connecting circuit ER. The terminal SRj of the cross-pointactivation conductor of the row must be understood as directly connectedto a marking output of the column logics circuitry in LOG since a singlecolumn and consequently a single control unit CB corresponds to a singleexternal network line connecting circuit. Apart from this modification,there is no difference at all between the cross-point control units ofthe subscriber's line circuits and of the mixed subscriber's line andexternal line connecting circuits.

An illustrative embodiment of an external network line connectingcircuit in a telephone equipment such as above described is shown inFIG. 4, wherein TR is the converter ensuring the two-wire to one-wireconversion of the conversation path from outside to inside, i.e., thematrix, terminal CR_(j). The winding of TR on the matrix side is shuntedby the same bidirectionnal shunts (DR) as is the converter T of thesubscriber's line circuits.

Across the wires of the incoming line LR, upstream of a relay contactK04 inserted in one of the wires, a call relay k03 is connected througha rectifier bridge RD. A unidirectional connection d34 ensures aprotective threshold against the low modulation frequencies of the voicecurrents. The relay K03 detects the calls to the circuit and its contactk03 is connected to an output terminal SL to the external call receiverpart of the logical control unit of the equipment. Across the wires LRare connected two relay coils K01 and K02 which are respectivelyactuated by opposite polarity currents, due to the series connections ofthe diodes d32 and d33 in their paths. Each relay is provided with awork contact which, when closed, marks the condition of the externalline at the respective terminals DS1 and DS2 to the scan of the logicalcontrol unit. It is necessary to provide these two relays because, atthe remote caller's set, the call current may be in either direction.

A protection circuit including a thermistance RI is inserted in one ofthe wires of the translator TR on the external line side.

The series contact k04 is a work contact of a relay K04 allotted to theseizure of the network and the sending of the dialling pulses to thenetwork line, incoming at CNj. A diode d35 shunts the coil of the relayK04 as a protection against accidental voltage surges from the coil.

What is claimed is:
 1. A telephone private switching equipmentcomprising a plurality of subscriber's line circuits, a plurality ofexternal network line connecting circuits, an intercommunication routingmatrix for selective interconnection of the circuits of the saidpluralities, said matrix being made of rows and columns havingcontrollable electronic switch cross-points, and a logical control unitincluding means for ensuring a cyclical scanning of the free, occupiedand communication request conditions of the circuits of the saidpluralities and means responsive to communication request conditionsdetected in said circuits for controlling selective activations ofelectronic cross-points of the matrix performing the establishment ofthe requested communications, wherein:each column of the matrixcomprises only a single conversation conductor and a single pointactivation conductor, each row of the matrix comprises only a singleconversation conductor and a single cross-point activation conductor,each cross-point of the matrix comprises a single electronic switchmember connected across the conversation conductors of a row and acolumn and having a trigger terminal connected by unidirectional leadsto the cross-point activation conductors of said row and said column,each subscriber's line circuit and each external network line connectingcircuit includes a two-wire to one-wire conversation converter havingits one-wire terminal connected to the conversation conductor of a rowin the matrix, each subscriber's line circuit includes means forperiodically applying pulses to the cross-point activation conductor ofthe row to which it is connected, and, each column of the matrix iscontrolled from a cross-point selection control unit having first andsecond terminals respectively connected to the cross-point activationconductor and to the conversation conductor of the column, and includingmeans for applying cross-point activation pulses to the first terminal,means for applying to said second terminal tone and switch biaselectrical currents and means for applying to said second terminal inactivated condition of a cross-point of the column a unidirectionalconstant voltage electrical current the path of which closes in thesubscriber's line circuit.
 2. Equipment according to claim 1, whereineach external network line connecting circuit is connected to a singlecross-point of the matrix.
 3. Equipment according to claim 1, whereineach cross-point of the matrix comprises a thyristor connected acrossthe conversation conductors of a column and of a row, having its triggerelectrode connected through a biassing resistor to the conversationconductor of the column and having said trigger electrode connected toboth cross-point activation conductors of the column and of the rowthrough a threshold member circuit responsive to the simultaneousactivitations of the said cross-point activation conductors. 4.Equipment according to claim 3, wherein each cross-point selectioncontrol unit includes means for temporarily applying a progressivelyincreasing voltage to the said second terminal before application of anactivation pulse on the said first terminal.
 5. Equipment according toclaim 1, wherein the said means for applying to said second terminal aunidirectional constant voltage electrical current includes meansresponsive to an order of cancellation of the established communicationfor progressively reducing the value of the voltage under which saidelectrical current is applied.
 6. Equipment according to claim 5,wherein the said means for applying to said second terminal aunidirectional constant voltage electrical current includes meansresponsive to the connection of a third party to the establishedconversation path through a pair of activated cross-points forcontrolling an increase of the value of the said electrical current. 7.Equipment according to claim 5, wherein the said cross-point selectioncontrol unit includes means responsive to the activation of across-point of the column to which said unit is connected for connectingto the said second terminal of the unit a dummy cross-point currentconsuming impedance and means responsive to the activation of a furthercross-point in the column for disabling said dummy cross-point currentconsuming impedance connecting means.
 8. Equipment according to claim 1,wherein each subscriber's line circuit includes the combination of meansfor temporarily storing an incoming call signal and for generating alocal tone call signal responsive to such storage, internal conditionmarking means having a terminal connected to the said circuit conditionscanning means; and local dialling means having an output connected tothe said terminal, and means for receiving from the comversationconductor of the row to which it is connected in the matrix a tonesignal of local dialling incitation.
 9. Equipment according to claim 8,wherein said subscriber's line circuit further includes means fortemporarily storing a signal requesting the subscriber to break thecommunication and for generating a local tone signal communication breakrequest.
 10. Equipment according to claim 1, wherein said subscriber'sline circuit means for periodically applying pulses to the cross-pointactivation conductor of the row to which it is connected comproses pulseforming means responsive to the application of each scan signal fromsaid circuit condition cyclical scanning means, consequently enablingactivation of any cross-point electronic switch connected to saidcross-point activation conductor of the row.